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July 31, 1928@ 1,679,095

G. w. PlcKARD v CLOSED-TUNED COIL OR LOOP AERIAL Filed June .'50, 1921 vs sheets-sheet 1 2e* Y 20 2 I9 2f L 7 z Hg, 2 f5 e 7 151 8 12 .5 2 J Je?July 31, 192s. 1,679,095

- G. W. PICKARD CLOSED TUNED COIL OR LOOP AERIAL Filed June 3o. 1921 ssheets-sheet 2 July 31, 1928. 1,679,095

G. w. PICKARD CLOSED TUNED COIL OR LOOP AERIAL Filed June 3o. 1921 3Sheets-'sheet 5` Patented July 3l, 1928.

UNITED STATES PATENT OFFICE.

GREENLEAF WHITTIER PICKAR-D, OF NEWTON CENTER, MASSACHUSETTS, .ASSIGNORITO WIRELESS SPECIALTY APPARATUS COMPANY, OF BOSTON, MASSACHUSETTS,

A CORPORATION OF NEW YORK.

Application iiled June 30, 1921. Serial No. 481,498.

i l5 are used extensively for direction finding, as

on ships and air planes, that is, for determining the location of adistant transmitter, and for the purpose of guiding and directing theship or air plane. Such loops are also used for receiving signals andmessages, because, by reason of their directional properties and sharptuning, static and interference are in a large measure eliminated4orreduced.

The theory of operation in receiving of such a .closed tuned circuit asdisclosed in my patent above-mentioned and disclosed in .thisapplication in an improved form` is briefly as follows. Theelectromagnetic Waves set up by the distant transmitter cut the closedtuned loop or receiving aerial in the same manner that the lines offorce generated around a primary of a transformer cut the turns of thetransformer secondary, setting up currents in the secondary circuit. Inlike manner, the electromagnetic Waves in cutting the loop aerial, Whichis tuned to the frequency of the transmitted Waves, set up oscillationsin the aerial. netic fiuxfwhich cuts the closed tuned aerial increaseswith its enclosed area or turn area. A loop or closed aerial operatingaccording to the above principle must be positioned with its plane inline With the distant transmitter in order to receive maximum signal orenergy. In a plane at right angles to the transmitter, the loop receivesno energy from the transmitter nor is it affected by it. The voltageinduced by the above flux in a closed radio loop of the above characterdepends upon the enclosed area or number of turns. However, it has beenfoundl that such a closed circuit has more sharply directional Theelectromagsiblefor optimum results in receiving or for directionfinding. Finally, as the voltage generated in the loop circuit increaseswith the number of turns or inductance, While the current decreasesinthe same proportion, I have found it advantageous to make the 'voltagelow and the current high. A loop circuit closed through a condenser doesnot have all of its capacity localized in this condenser. By reason ofthe inevitable proximity of earth"A and other conducting objects, acertain amount of electrostatic capacity results from the conductor or'conductors of the loop to its surroundings. If, as ina multiturn loop,'the condenser is of small value (because of the large inductance of amultiturn loop), this external electrostatic field may be relativelylarge, and if the disposition of surrounding object.` is notsymmetrical, the directional properties of the loop are impaired. If theloop be made of minimum inductance, that is, a single turn, the capacityof the condenser becomes large as compared with the externalelectrostatic field, and the disturbing effect of surrounding objects isminimized.

In using the radio compass or direction finder, it is an advantage,however, to have wthe loop aerial receiving set of minimum volume andextension. obtained in present-day radio Compasses and direction findersby providing multiple turns instead vof a single turn, wherebyelectromotive forces developed in each turn by the cutting of themagnetic waves are added together to produce an increased E. M.\F. inthe loop for actuating the receiving apparatus or detector. Such asystem, however, is inefiieient electrically, and it also is notpossible `to obtain sharply directional properties on account of theSmall capacity neces-I This compactness isy sitated by the highinductance of the multiturn loop, as above explained. Also, theincreased resistance due both to the increased length of wire necessaryfor the multiple turns, and the increased high frequency resistancecaused by the adjacent turns is wasteful of energy.

In such direction finders as are now in use, the energy or oscillationsset up in the coil are detected by means of a detector circuit connectedin shunt with the closed aerial and in shunt with the condenser inseries with the aerial in the manner disclosed in Figure 2 of the abovepatent or occasionally by way of an inductive coupling such as atransformer having an inductance coil in series with the loop andinductance coils in series with the receiving circuit, as shown inFigure 3 of my patent. These two modes of connection, while completelyoperative, have involved certain disadvantages such as decreased signalreception through high resistance and a certain ioss in sharpness ofdirectional characteristics by reason of the high loop inductance, asexplained above,

this latter disadvantage bein(r uite serious' when the loop aerial isused as' a direction finder or radio compass.

en object of the invention is to provide a loop aerial in which theefficiency and directional properties are increased.

Another object of the invention is to provide a loop aerial and detectorcircuit in which the electrical connection between the two obviates theuse of long leads or resistance-introducing coupling coils such as haveheretofore been used.

Janother object of the invention is to provide a closed loop aerial andreceiving detecting circuit in which electrostatic troubles between theloop, surrounding objects and the receiving circuit to the detriment ofthe directional properties and efficiency of the loop are obviated.

Another object of the invention is to provide a compact, simpleconstruction of few parts which may be easily handled and operated.

In the accompanying drawings, I have illustrated embodiments of myinvention in which j Figure 1 is an elevation with parts in section ofone form of the invention;

Fig. 2 is a sectional plan view on the line 2 2 of Fig. 1, looking inthe direction of the arrows and on an enlarged sca-le;

Fig. 3 is an elevation of a modiiied form of the invention;

Fig. 4 is a plan view thereof;

Fig. 5 is a detail sectional plan view on the line 5-5 of Fig. 3,looking in the direction of the arrows;

Fig. 6 is a sectional elevat-ion of another form of the invention,illustrated diagrammatically; and

Fig. 7 is an elevation, partly in section, of a further embodiment ofthe invention, illustrated diagrammatically.

\ Referring to Figs. l and 2, I have illus- `trated a closed aerialcircuit 1 which is completed through a static condenser system 2. Thiscondenser system, which is in series with the closed loop, is made(because of the low inductance of the loop of the present invention,) oftwo parts 2Ll and 2h'in parallel with each other and both connected orconnectible in series with the loop 1. rl`he part 2 comprises a numberof condensers, preferably mica condensers, each having differentcapacities and one and all being capable of being connected in serieswith loop 1. 'I he part I2" is a variable condenser, and after theapproximate resonant capacity value of the condensers 2a is obtained,the iinal tuning adjustment may be secured by the variable condenser 2l.

'Ihe condenser system 2 comprises a metal casing C, supporting itheaerial' 1, and provided at the bottom with a vertical tubular extension3, rotatably supported yin frame 4, ball bearings 5 being provided foreasy turning, whereby the entire apparatus, including the casing C,condensers 2 and the loop 1, may be rotated about a vertical axis forthe purpose of properly positioning the same with reference to thedistant transmitter. rIhe lower end of the tube or sleeve 3 is providedwit-h an adjusting wh-eel or handle 6. rIhe loop 1 is supported oncasing C and insulated therefrom by bushings 7 of any suitableinsulating material having good dielectric properties such as bakelitedileeto or quartz, the bushings being arranged at diametrically oppositepoints in easing 2. 'Ihe free ends ot the loop 1 are inserted withinthese bushings 7 and project inside of the casing C. One end of the loopis provided with a metal segment 8 to which are electrically connectedadjacentjends of the condensers 2a and 2". 'lelescoping the tube 3 is asleeve 9 provided at its lower end with an operating wheel or handle 10and terminating at its upper end within casing2 in an insulating plate110i suitable insulating and dielectric properties. Secured to the uppersurface of the insulating plate 11 is a met-al plate or segment 12adapt-ed to engage contact lingers 13 (Fig. 2) upon the condensers 2a atthe ends thereof remote fromtheir points of connection to the segment 8.A contact finger 14 also engages the plate l2 and is electricallyconlnectedthrough conductor 15 within casing C to the opposite end ofthe loop, whereby the circuit 1 is closed through one or niore of thecondensers 2a. Extending vertically1 through the sleeve 9 is anactuating rod 16 provided at its lower end with an operating handle 17for operating the variable condenser 2", The upper end 18 of the rod 16l line with the distant transmitter receives'v has secured to it therelatively adjustable plates 19 of Ythe variable condenser 2", which areinterposed between the relatively fixed plates-20 of the variablecondenser. The plates 20 are electrically connected to segment 8, whilethe plates 19 are insulated from the rod 16 by suitable insulationr 21.

The relatively movable plates 19 are electrically connected by aconductor 22 to conductor 15.

By the above construction, it will be seen that in tuning the closedloop l, oneormore of the condcnscrs 2a may bc successively thrown intoseries with the loop 1 by means ot' the actuating `handle 10 forsecuring rough adjustment and tuning of the circuit 1 and the inalltuning and adjustment may be secured by rotating handle 17, therebyactuating and varying the variable condenser 2". By means of the handle6, the entire apparatus may be properly positioned. A coill 23, whichwith aerial 1 as a whole constitutes a transformer and which maycomprise a number of turns depending on given conditions, is secured bymeans ,of an adjustable collar 24 to the tube 3, this collar beingadjustably clamped to tube `3 by any suitable clamping means 25. Byrotating the collar 24 around the sleeve 3 and thereby rotating coil 23,the inductive coupling between coil 23 and loop 1 may be varied foroptimum'eifect. The-leads 26 from coily .23 are electrically connectedto any suitable detecting or transmitting circuit, such as, forinstance, an audion circuit, crystal detector circuit or any standardtransmitting circuit, whereby the oscillations set up in the loop l areinductively transferred to transformer secondary 23 and detected orwhereby the reverse occurs on transmitting.

The coil aerial 1 in the present invention consists preferably of asingle turn of low resistance conductor, or, if so desired, may consistof more than one turn.l If the loop be made of a single turn conductor,the most efficient form for this conductor is a circle of thin-walledcopper tubing, as in Fig. 1, while for a loop of more than one turn asuitably stranded or braided cable should be used (Figs. 6 and 7) tokeep the high fre quency resistance low. In all cases, the loop shouldenclose suilicient area to be operative under any given set of`conditions. By way of example only, such a ring or closed loop may havea diameter across the same of three feet, the diameter of the tubularpart being about 2 inches, and the thickness of walls about 35 of aninch. Such. construction, when properly tuned by the condenser system 2,constitutes a closedtuned circuit.

enclosing an area whiclujvhen its plane is in maximum current, and whenat right angles to the transmitter, has 'a null point. The copper tubingloop such as disclosed 1n Figs,

1 and 2 has minimum resistance, minimum inductance, maximum current andmaximum capacity.

A single turn loop such as disclosed in Figs 1 and 2 provides a closedcircuit havmg highr current and comparatively low voltage. This circuit1 as a whole, by reason of the oscillations impressed uponl it by theelectromagnetic waves, acts in receiving as a primary of a transformerwhich is in inductive` relation to the secondary of the transformer 23secured to the tube 3 below it. Tlns secondary 23 is connected up to anysuitable receiving and detecting circuit as described above. Thesecondary 23 is adjustably secured to sleeve 3, and can be adjustedaround sleeve 3 as an axis to vary the coupling between it and coil 1.In the use of the apparatus, the coil 23 is adjusted until maximumresponse or optimum respense or signal is pbtained. Thereafter, by meansof the clamping screw 25, the coil 2 3'is fixed relatively to the loop1, and the whole, both coil 23 and loop 1, may be ro.- tated inaccordance with direction-finding practice.

' An advantage of the type of coupling between the closed aerial 1 andthe coil 23 resides in the fact that the coils l1 and 23 may bephysically separated suiiicicntly so that there will be no electrostaticcoupling or capacity eect between them which impairs the etliciencyof/the apparatus as a direction finder. The optimum coupling between areceiving aerial circuit and' the 'detector or secondary circuit dependsupon the `resistance or damping of these circuits, and where theresistance, as in my invention herein, is

very low, the optimum coupling is also very low, thereby permitting aconsiderable physical separation between the circuits, which in turnminimizes the electrostatic effect. In constructions now in use in whichthe receiving circuit is connected by means of an inductive coupling inseries with the loop l or by leads such as above referred to, forming adetector shunt with the loop, the electrostatic effects are detrimentalto the efficiency of the circuit, because of the close couplingnecessitatedby the relatively high circuit resistance. y

As is well known, the coupling between two coils of a transformer can beadjusted either by increasing the parallel separation between the coils,or by moving the coils transversely ,to cach other, or by rotatingonecoil around an axis, whereby the angular position between the coils isvaried. lVhen the planes are parallel, we have maximum,l mutualinductance, and when the planes are at right angles, no mutualinductance. mum position or coupling between the coils which can beeasily determined bythe o er- 'ator for obtaining the loudest signal.

his

However, there is always an ,opti-l optimum coupling under any conditioncan be obtained with the two coils 1 and 23 spaced sufficiently apart toavoid electrostatic coupling.

Such an arrangementas 'above described,'

in which the closed aerial 1 as a whole constitutes a primary of anoscillation transformer and the coil 23 constituteswthe secondary 'ofthe transformer,l materially -lowers the losses in the completeapparatus and ensures better tuning and efficiency. The decreasing ofthe resistance and inductance in the coil 1 ensures increased currentand obviates the necessity for a large number of turns in coil`1. Thenumber of secondary turns of the transformer 23 is large compared to thenumber of turns of the closed aerial whereby the necessary voltage isdeveloped for operating the detector. In other words, in receiving, thecoil 1 and the secondary23 constitute a step-up transformer.

This being the case, there is no necessity for a large number of turnsin the loop -1 to generate the necessary voltage to operate thedetector, this increased voltage being secured in the secondary 23 bythe present invention. An advantage of the construction shown lin lFig.1 is that the receiving circuit is of V`very low resistance because nocoupling coil is used, nor are there any long leads. This permits theuse of a single turn receiving coil, hence giving maximum efficiency.Again, the directional properties of the system are extremely sharp,because the closed loop circuit 1 has no long leads or connections toany more or less grounded circuit,

as isthe case in the ordinary modes of communication. Furthermore, thesystem is extremely simple because it is not necessary, as in the caseof inductive couplings, to insert an inductance in series with the loop,nor is it necessary to provide long leads to the detector circuit. j

As illustrated in Fig. 1, both the closed aerial or primary and thesecondary coil 23 are mounted on the same frame, and if the apparatus isused on a singlewave-length, .the coupling may be made non-adjustableand fixed by the constructor at the best value. Of course, if used onwide wave-length ranges, the coupling between the coils 1 and 23 shouldbe made variable in any suitable manner such as those enumerated above.

In Figs. 3, 4 and 5, I have illustrated a modified embodiment of ltheinvention in which the number of movable and adjustable parts is reducedto a minimum and i which is especially adapted for larger aerials whichcannot be conveniently rotated. In this form, I have illustrated aclosed loop or circuit 30 having in series therewith an adjustabletuning condenser 31 and a second closed loop or circuit 32 having inseriesy therewith an adjustable tuning condenser 3 3. These two loops 30and 32 may be secured 1n Vlustrated, the loops being substantiallycircular and ofthe same size, havino' similar electricalcharacteristics, insulated fiom each other, and arranged-symmetricallyrelative to each other. The closed aerials 30 and 32 may comprise anvsuitable conductive material or any number of turns, but, for purposesof illustration, I have shown the same as being composed of my tubularconstructions such as disclosed in Fig. 1'. Extending vertically alongthe line of intersection ofthe two loops 30 and 32 is a rod 35,journaled at 36 in the support 34 and having a wheel or handle 37 at itslower end. Secured to rod 35 at its upper end is a coil 38 comprisingany suitable or sufficient number of turns for the purposes of thisinvention. This coil 38 is arranged with its axis on a line coincidingwith the line of intersection of the planes of the two loops 30 and 32and is arranged centrally and symmetrically of the two loops. Leads 39from the coil 38 may be connected to any suitable receiving or de-.,tector circuit as above-described or a transmitting circuit. Securedto rod 35 above the handle 37 is a pointer 40 adapted to play over adial 41 whereby the direction may be determined.

Electrical oscillations set up from a transj above construction, I amenabled to obtain direction without necessitating the rotation of theaerial or aerials. At the same time, the necessity for varying theinductive coupling between lthe loop and the coil and at the same timerotating both the loop aerial vand secondary coil is obviated. Theprinciple of the invention disclosed in Figs.v 3 to 5 is the same asthat disclosed in Fig. 1, that is to say, the aerials 30 and 32 as awhole upon receiving constitute the primary of a transformer, while thecoil 38 constitutes the secondary of said transformer.

In Fig. 6, I have illustrated another embodiment of the inventioncomprising a closed aerial or loop 45, which may consist of severalturns of stranded wire wound upon a suitable insulating frame 46 and inseries with which is an adjustable tuning condenser 47. Located withinloop 45 is a secondary coil 49 consisting of suitable nurnber of turnsof wire, preferably exceeding the number of turns in aerial 45, beingthe I or loo -with coil 49 constitutes a transformer functioningaccording to the principles of the invention described above. v

In Fig. 7, I have illustrated a form of my However, in the presentembodi- The coil 49 may be connected,

aerial circuit, coupled inductively to a coil of a detecting ortransmitting circuit, the

two constituting the primary of an oscillation transformer.

A form of the presentv invention which I have used consisted of33 turnsof 378 inch Belden highl frequency cable wouhd as a single layersolenoid on asquare frme 44 inches on a side. This loop aerial i'th itsassociated tuning condenser, and when cou-y pled to'a detector circuitfor maximum signal, had the extremely low resistance of 1.2 ohms atavwave length of 12,500 meters. Good signals were received from Europeanstations, and, owing to the low resistance of and secondary invent`onslightly modified from that of Fig. st he .loop aerial, very sharptuning was POS- 6, which comprises the closed tuned primary aerial 51comprising several turns tuned y a' variable condenser 52 in series withit. Cupled with this loop aerial'is a secondary coil 53y f a sufficientnumberl of turns. Any desire variation of coupling may be obtained byeither moving the coil 53 axially away or towards the coil 51, or, asindicated by the double-headed vertical arrow, coil 53 may be eithermoved laterally across, the face of coil 51 or rotated around an axisparallel with the -double-headed ar row, to vary the coupling. o y

The secondary circuit of Fig. 7 including coil 53 is tuned by a variablecondenser 54, and theradio frequency amplifier 55 is connected betweenthis secondary circuit and the indicator 56. In the form of Fig. 7, thediameter of the secondary 53 approximates or is of the order ofmagnitude of that of the coil aerial 51, and such secondary 53, inasmuchas it is tuned to the frequency ofthe primary 51, acts as a closed tunedcoil aerial to a certain extent, the current set up in coil 53 by thetransmitted electromagnetic waves being added in quadrature to thecurrents transferred by induction from aerial 51 to coil 53.

The main features of the above-described constructions reside in thefact that the inductance and resistance of the closed aerial circuit isreduced to a minimum by reason of the omission of conductive leads andinductances inserted inseries or in shunt in the circuit. The secondarycoil with which the primary or closed loop aerial co-operates as a wholeis suiiiciently spaced to prevent electrostatic couplings, but at thesame time is so adjustably coupled to the closed aerial as to receivemaximum or optimum signal. This arrangement increases the sliarpness'ofdirectivity by an appreciable amount. By the above construction,increasedicurrent is secured in the coil aerial, which current istransferred directly from the coil aerial to a sible.

.It is to be understood that the invention is not limited-to theembodiments and features specifically shownand described herein, butthat such embodiments and features are subject to changes' andmodifications' denser connectibly in series therewith,-

means for rotating said aerial, local operating apparatus, meansextending alonoF the axis of rotation of said aerial for a justing saidcondenser, and a circuit inductively associated with the loop as atransformer secondary to the loop as a primary; the loop beinglinsulated from the local uapparatus and the secondary being electricallyconnected to the local apparatus.

2. In a radio apparatus of the character described, a closed loop oftubular metal having a condenser in series therewith, and havingsufficient area to act as an aerial, said loop constituting the primary'of a transformer, a rotatable metal tube on lwhich said loop ismounted, and from which its ends are insulated, said condenser alsobeing carried by said rotatable tube, leads controlling said condenserand extending` through said rotatable tube; and the coil when in linewith the distant transmitter,

and means for detecting the oscillations set up in said loop by theelectromagnetic waves set up by said distant transmitter, whichcomprises a secondarycoil in inductive relation to said loop, .andmounted upon the pivot of said loop, said secondary coil beingadjustably mounted on said pivot as an axis independent of said loop,and' both vcoil and loop being rotatable together around said loop axisfor direction finding 5 purposes.

4. In radio apparatus ofthe character described, a single turn closedaerial of tubular metal having a, plurality of oondensers connectible inseries therewith, means for rotating said aerial around a Vertical axis,a plu- 10 rality of condenser adjusting devices arranged along the axisof said aerial, and a multiturn' coil adjustable on the said axis,coupled t'said aerial and connected in a circuit, said coil and aerialconstituting a trans 15 former.

`GREI'INLEAF WHlTTlER PICK'ARD.

